Better Bombs Through Crystalline Chemistry

Using a crystallization process favored by the drug companies, University of Michigan chemists are refining the explosives the U.S. military already uses and making hybrids that are less prone to accidental detonation.

Borrowing a trick from the pharmaceutical industry, scientists led by chemist Adam Matzger of the University of Michigan have combined the powerful explosive CL-20 with the standard high melting-point explosive (HMX) used by the military to create a hybrid explosive that could be better than either—with as much oomph as HMX, but not as delicate and prone to accidental detonation as CL-20.

Matzger says research into new explosive materials tends to focus on finding entirely new chemistries. But his team instead devised a way to reimagine the explosives already in use. The researchers combined CL-20 and HMX using a technique called co-crystallization, in which two different materials join at a molecular level to form crystals.

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Co-crystallization has attracted a lot of attention in the pharmaceutical industry because it can alter the solubility of a drug without changing its chemical makeup. Increased solubility means it can be more easily absorbed into the body, so in theory drugs can be made more effective simply by co-crystallizing them with a suitable partner. There are several ways to make co-crystals, but the simplest version involves dissolving a mixture of the two materials, in the correct ratio, in a solvent. Then, when the solvent evaporates, the crystals form naturally. The difficulty comes with knowing what sort of crystals will be formed. Matzger says that because there has been less research on explosives compared to pharmaceuticals, and they do not form bonds in the same way, his team couldn't simply copy the techniques used for drugs.

For their first project, Matzger and his colleagues worked with TNT because its chemistry is so well–known. The team co-crystallized TNT with various other substances and produced a new material they nicknamed Jett. The results confirmed the theoretical predictions, proving that explosives could be combined with other materials. However, because the explosive had been diluted with non-explosive material, Jett was less powerful than TNT.

The next step, then, was to co-crystallize two different explosives. One component would be HMX, otherwise known as Octogen, a standard military explosive. The other was CL-20, which derives its name from being developed by the Naval Air Weapons Station at China Lake. CL-20 is more powerful than HMX, but is not used in any current munitions because it is too sensitive and prone to being detonated by accident.

But since the team knew much less about the chemistry involved in combining these explosives, they couldn't be sure which mixture would be best, what type of crystals would form, and how explosive the resulting material would be. "We might get accused of being Edisonian in our approach," Matzger says, referring to the inventor's penchant for trial and error.

After trying various ratios of CL-20 and HMX, Matzger found that when mixed two parts to one they could combine into a new co-crystalline explosive that he calls Chaz. The team produced Chaz only in milligram quantities (a fraction the size of a grain of rice), but it produced an impressive explosion in detonation tests.

Detailed analysis showed that Chaz has a detonation velocity—one of the key factors in the power of an explosive—significantly greater than the purest form of HMX, though somewhat less than pure CL-20. Crucially, however, tests show that the new material is as stable as HMX and not overly sensitive like CL-20. The two-to-one ratio also gives Chaz a better oxygen balance than HMX, Matzger says, so it burns more cleanly.

The next step will be to scale up the production process, a job to be carried out at Sandia National Laboratories or a similar organization that's used to working with large quantities of these materials. Because HMX and CL-20 are already manufactured, all that is needed is the final production of the co-crystals. An industrial partner, Nalas Engineering, has also expressed an interest. (Matzger's development work was sponsored by the Defense Threat Reduction Agency.)

New mixtures like Chaz may result in better munitions for attacking deeply buried targets such as nuclear and chemical weapons facilities. Destroying these targets requires explosives that are powerful, but resistant to the initial shock of impact. (If the bomb explodes on the surface, it won't destroy the bunker.)

Meanwhile, Matzger's team is already working on its next project. He can't say what it is, but it involves co-crystallization and explosives, and he's excited by the possibilities.

"There are not many fields where you can go into the laboratory in the morning, mix two materials together, and come up with a game-changer," he says.